Methods of preparing three-dimensional, macroscopic assemblages of carbon fibrils and the products obtained thereby

ABSTRACT

A method of making randomly oriented carbons fibrils having a cylindrical constant diameter with c-axes perpendicular to their cylindrical axis, which are free of pyrolytically deposited carbon, with a diameter of between 3.5 and 70 nanometers, by dispersing carbon fibrils in a medium and separating them from the medium, by filtration or evaporation to form a porous mat or sheet.

This application is a division of application Ser. No. 08/428,496(Docket No. 370077-3130), filed Apr. 27, 1995, which is a continuationof application Ser. No. 08/057,328, filed May 5, 1993 now abandoned.

FIELD OF THE INVENTION

The invention relates generally to assemblages of carbon fibrils. Morespecifically, the invention relates to three dimensional, macroscopic,assemblages of randomly oriented carbon fibrils having a bulk density offrom 0.001 to 0.50 gm/cc and to methods for preparing such assemblages.Even more specifically, the invention relates to such assemblages foruse as catalyst supports, electrodes, chromatographic media, etc. and tocomposite structures comprising the assemblage and a second materialcontained within the assemblage.

BACKGROUND OF THE INVENTION

Carbon fibrils are vermicular carbon deposits having diameters less than500 nanometers. They exist in a variety of forms, and have been preparedthrough the catalytic decomposition of various carbon-containing gasesat metal surfaces.

Tennent, U.S. Pat. No. 4,663,230, describes carbon fibrils that are freeof a continuous thermal carbon overcoat and have multiple graphiticouter layers that are substantially parallel to the fibril axis. As suchthey may be characterized as having their c-axes, the axes which areperpendicular to the tangents of the curved layers of graphite,substantially perpendicular to their cylindrical axes. They generallyhave diameters no greater than 0.1 micron and length to diameter ratiosof at least 5. Desirably they are substantially free of a continuousthermal carbon overcoat, i.e., pyrolytically deposited carbon resultingfrom thermal cracking of the gas feed used to prepare them.

Tubular fibrils having graphitic layers that are substantially parallelto the microfiber axis and diameters between 3.5 and 75 nanometers, aredescribed in Tennent et al., U.S.S.N. 871,676 filed Jun. 6, 1986,refiled as continuation application Ser. No. 593,319 filed Oct. 1, 1990,now U.S. Pat. No. 5,165,909, issued Nov. 24, 1992 ("Novel CarbonFibrils, Method for Producing Same and Compositions Containing Same"),Tenant et al., U.S.S.N. 871,675 filed June 6, 1986, refiled ascontinuation application Ser. No. 492,365 filed Mar. 9, 1990, now U.S.Pat. No. 5,171,560, issued Dec. 15, 1992 ("Novel Carbon Fibrils, Methodfor Producing Same and Encapsulated Catalyst"), Snyder et al., U.S.S.N.149,573 filed Jan. 28, 1988, refiled as continuation application Ser.No. 494,894, filed Mar. 13, 1990, refiled as continuation applicationSer. No. 694,244, filed May 1, 1991 ("Carbon Fibrils"), Mandeville etal., U.S.S.N. 285,817 filed Dec. 16, 1988, refiled as continuationapplication Ser. No. 746,065, filed Aug. 12, 1991, refiled ascontinuation application Ser. No. 08/284,855, filed Aug. 2, 1994("Fibrils"), and McCarthy et al ., U.S.S.N. 351,967 filed May 15, 1989,refiled as continuation application Ser. No. 823,021, refiled ascontinuation application Ser. No. 117,873, refiled as continuationapplication Ser. No. 08/329,774, filed Oct. 27, 1994 ("Surface Treatmentof Carbon Microfibers"), all of which are assigned to the same assigneeas the present application and are hereby incorporated by reference.

Fibrils are useful in a variety of applications. For example, they canbe used as reinforcements in fiber-reinforced composite structures orhybrid composite structures (i.e. composites containing reinforcementssuch as continuous fibers in addition to fibrils). The composites mayfurther contain fillers such as a carbon black and silica, alone or incombination with each other. Examples of reinforceable matrix materialsinclude inorganic and organic polymers, ceramics (e.g., lead or copper).When the matrix is an organic polymer, it may be a thermoset resin suchas epoxy, bismaleimide, polyamide, or polyester resin; a thermoplasticresin; or a reaction injection molded resin.

OBJECTS OF THE INVENTION

It is an object of the invention to provide a composition of matterwhich comprises carbon fibrils and more specifically an assemblage ofrandomly oriented carbon fibrils which has a low bulk density and whichcan be used as a substrate or medium for various industrial andscientific purposes.

It is another object of the invention to provide a composition of mattercomprising a three-dimensional, macroscopic assemblage of a multiplicityof randomly oriented carbon fibrils having a low bulk density to whichcan be added one or more functional second materials in the nature ofactive catalysts, electroactive species, etc. so as to form compositeshaving novel industrial properties.

It is yet another object of the invention to provide three-dimensional,macroscopic assemblages of a multiplicity of randomly oriented carbonfibrils which have isotropic physical properties so that suchcompositions can be used reliably and interchangeably for multipleindustrial purposes.

It is a further object of the invention to provide processes for thepreparation of such three-dimensional microscopic assemblages of carbonfibrils which are efficient and convenient to use in the preparation oflow-density compositions.

It is a still further object of the invention to provide improvedcatalyst supports, filter media, chromatographic media, EMI shieldingand other compositions of industrial value based on three-dimensionalassemblages of carbon fibrils.

SUMMARY OF THE INVENTION Definitions

The term "assemblage" refers to any configuration of a mass ofindividual fibrils and embraces intertwined as well as discrete fibrilembodiments.

The term "macroscopic" means that the assemblages may be of any suitablesize to achieve an industrial or scientific purpose.

The term "physical property" means an inherent, measurable property ofthe assemblage, e.g. resistivity.

The term "isotropic" means that all measurements of a physical propertywithin a plane or volume of the assemblage, independent of the directionof the measurement, are of a constant value. It is understood thatmeasurements of such non-solid compositions must be taken on arepresentative sample of the assemblage so that the average value of thevoid spaces is taken into account.

The term "relatively" means that ninety-five percent of the values ofthe physical property when measured along an axis of, or within a planeof or within a volume of the assemblage, as the case may be, will bewithin plus or minus fifty percent of a mean value.

The term "substantially" means that ninety-five percent of the values ofthe physical property when measured along an axis of, or within a planeof or within a volume of the assemblage, as the case may be, will bewithin plus or minus ten percent of a mean value.

The terms "relatively isotropic" and "substantially isotropic"correspond to the ranges of variability in the values of a physicalproperty set forth above.

The Invention

The invention is broadly in a composition of matter consistingessentially of a three-dimensional, macroscopic assemblage of amultiplicity of randomly oriented carbon fibrils, said fibrils beingsubstantially cylindrical with a substantially constant diameter, havingc-axes substantially perpendicular to their cylindrical axis, beingsubstantially free of pyrolytically deposited carbon and having adiameter between about 3.5 and 70 nanometers, said assemblage having abulk density of from 0.001 to 0.50 gm/cc.

The assemblages described above can be used to great advantage asthree-dimensional matrixes for a number of industrial purposes. Forexample, the assemblages can be used as filter media, as catalystsupports, as electroactive materials for use, e.g. in electrodes in fuelcells and batteries, and as chromatography media. It has been found thatthe assemblages are useful in the formation of composites which comprisethe assemblage together with either a particulate solid, anelectroactive component or a catalytically active metal ormetal-containing compound, as well as in composites with polymers.

It has now been found that highly advantageous three-dimensional,macroscopic assemblages of randomly oriented carbon fibrils can beprepared which have relatively uniform physical properties along one,preferably two and most desirably three-dimensional axis of thethree-dimensional assemblage. Preferred compositions prepared accordingto the methods of the invention have uniform physical properties alongat least one dimensional axis and have relatively isotropic physicalproperties in at least one plane of the assemblage and most desirablyare isotropic throughout the entire three-dimensional structure.

These advantageous compositions can be prepared by dispersing fibrils inaqueous or organic solid media and then filtering the fibrils. Lowdensity compositions are advantageously prepared by forming a gel orpaste of carbon fibrils in a fluid, e.g. an organic solvent such aspropane and then heating that gel or paste to above the criticaltemperature of the medium, removing supercritical fluid and finallyremoving a low-density porous mat or plug from the vessel in which theprocess has been carried out.

DETAILED DESCRIPTION BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph of a three dimensional assemblage ofrandomly oriented carbon fibrils prepared by the method of EXAMPLE 1.

PRODUCTION OF CARBON FIBRILS

Fibrils are prepared by contacting a carbon-containing gas with a metalcatalyst in a reactor for an appropriate period of time, at a suitablepressure, and at a temperature sufficient to produce fibrils with theabove-described morphology. Reaction temperatures are generally400°-850° C., more preferably 600°-750° C. Fibrils are advantageouslyprepared continuously by bringing the reactor to the reactiontemperature, adding metal catalyst particles, and then continuouslycontacting the catalyst with a carbon-containing gas.

Examples of suitable feed gases, catalysts and reaction conditions aregiven in the several patent applications referenced above as well as inMoy et al., U.S. patent applications Ser. Nos. 887,307, filed May 22,1992, refiled as continuation application Ser. No. 08/284,742, filedAug. 2, 1994, refiled as continuation application Ser. No. 08/469,430,filed Jun. 6, 1995 and 887,314 filed May 22, 1992, refiled ascontinuation application Ser. No. 07/320,564, filed Oct. 11, 1994, whichare hereby incorporated by reference.

Fibrils may be prepared such that at least a portion of the fibrils arein the form of aggregates. As used herein, an aggregate is defined astwo or more entangled fibrils. Fibril aggregates typically havemacroscopic morphologies, as determined by scanning electron microscopy,in which they are randomly entangled with each other to form entangledballs of fibrils resembling a bird's nest ("BN"); or as aggregatesconsisting of bundles of straight to slightly bent or kinked carbonfibrils having substantially the same relative orientation, and havingthe appearance of combed yarn ("CY") e.g., the longitudinal axis of eachfibril, despite individual bends or kinks, extends in the same directionas that of the surrounding fibrils in the bundles; or, as aggregatesconsisting of straight to slightly bent or kinked fibrils which areloosely entangled with each other to form an "open net" ("ON")structure. In open net structures the degree of fibril entanglement isgreater than observed in the combed yarn aggregates (in which theindividual fibrils have substantially the same relative orientation) butless than that of bird's nest.

In addition to fibrils such as are described in Tennent, U.S. Pat. No.4,663,230, fibrils may be prepared having different macromorphologies,such as the so-called fishbone ("FB") morphology described in publishedEuropean Patent Application No. 198,558 to J. W. Geus (published Oct.22, 1986). Fibrils of the so-called fishbone morphology may becharacterized as having their c-axes (as defined above) at some angleless than perpendicular to the cylindrical axes of the fibrils. Theinvention relates to such fishbone fibrils as well as to those describedin Tennent, U.S. Pat. No. 4,663,230.

Carbon Fibrils

The carbon fibrils preferably comprise a combination of discrete fibrilsand fibril aggregates. However, the fibrils may all be in the form ofaggregates. The aggregates, when present, are generally of the bird'snest, combed yarn or open net morphologies. The more "entangled" theaggregates are, the more processing will be required to achieve asuitable composition. This means that the selection of combed yarn oropen net aggregates is most preferable for the majority of applications.However, bird's nest aggregates will generally suffice.

The Assemblages

Broadly, the invention is in a composition of matter consistingessentially of a three-dimensional, macroscopic assemblage of amultiplicity of randomly oriented carbon fibrils, said fibrils beingsubstantially cylindrical with a substantially constant diameter, havingc-axes substantially perpendicular to their cylindrical axis, beingsubstantially free of pyrolytically deposited carbon and having adiameter between about 3.5 and 70 nanometers, said assemblage having abulk density of from 0.001 to 0.50 gm/cc. Preferably the assemblage hasrelatively or substantially uniform physical properties along at leastone dimensional axis and desirably have relatively or substantiallyuniform physical properties in one or more planes within the assemblage,i.e. they have isotropic physical properties in that plane. In otherembodiments, the entire assemblage is relatively or substantiallyisotropic with respect to one or more of its physical properties.

The physical properties which can be easily measured and by whichuniformity or isotrophy are determined include resistivity and opticaldensity.

Composites Containing the Assemblages

Broadly, the fibril assemblages may be used for any purpose for whichporous media are known to be useful. These include filtration,electrodes, catalyst supports, chromatography media, etc. In addition,the assemblages are a convenient bulk form of carbon fibrils and maythus be used for any known applications including especially EMIshielding, polymer composites, active electrodes, etc.

For some applications like EMI shielding, filtration and currentcollection, unmodified fibril assemblages can be used. For otherapplications, the fibril assemblages are a component of a more complexmaterial, i.e. they are part of a composite. Examples of such compositesare polymer molding compounds, chromatography media, electrodes for fuelcells and batteries, fibril supported catalyst and ceramic composites,including bioceramics like artificial bone.

In some of these composites, like molding compound and artificial bone,it is desirable that the non-fibril components fill--or substantiallyfill--the porosity of the fibril assemblage. For others, likeelectrodes, catalysts, and chromatography media, their usefulnessdepends on the composite retaining at least some of the porosity of thefibril assemblage.

Methods of Preparing Fibril Assemblages

While fibrils of any morphology may be used to prepare the assemblagesof the invention by using the methods of the invention, it is preferredto use fibrils having a parallel type morphology such as CC, DD or CY.Methods for the preparation of fibrils having these morphologies aredescribed in Moy et al., U.S. patent application Ser. Nos. 887,307 and887,314 filed May 22, 1992.

Mats with a thickness between 0.02 and 0.50 millimeters have a densityof typically 0.20 g/cc corresponding to a pore volume fraction of 0.90.Their electrical resistivity in the plane of the mat is typically 0.02ohm/cm; resistivity perpendicular to the mat is typically 1.0 ohm/cm.

Solid ingredients can be incorporated within the fibril mat by mixingthem with the fibril dispersion prior to mat formation. The content ofother solids in the dry mat may be made as high as fifty parts solidsper part of fibrils.

Fibrils from the synthesis reactor are dispersed at high shear in ahigh-shear mixer, e.g. a Waring Blender. The dispersion may containbroadly from 0.01 to 10% fibrils in water, ethanol, mineral spirits,etc. This procedure adequately opens fibril bundles, i.e. tightly woundbundles, of fibrils and disperses fibrils to form self-supporting matsafter filtration and drying. The application of high shear mixing maytake up to several hours. Mats prepared by this method are not free ofaggregates.

If the high shear procedure is followed by ultrasonication, dispersionis improved. Dilution to 0.1% or less aids ultrasonication. Thus, 200 ccof 0.1% fibrils may be sonified by a Bronson Sonifier Probe (450 wattpower supply) for 5 minutes or more to further improve the dispersion.

To achieve the highest degrees of dispersion, i.e. a dispersion which isfree or virtually free of fibril aggregates, sonication must take placeeither at very low concentration in a compatible liquid, e.g. at 0.001%to 0.01% concentration in ethanol or at higher concentration e.g. 0.1%in water to which a surfactant, e.g. Triton X-100° has been added in aconcentration of about 0.5%. The mat which is subsequently formed may berinsed free or substantially free of surfactant by sequential additionsof water followed by vacuum filtration.

The three-dimensional, macroscopic assemblage may be a compositecomprising a particulate material selected from aluminum oxide, silicondioxide or silicon carbide. The composite may also contain anelectroactive component selected from lead, lead compounds, manganese ora manganese compound.

Particulate solids such as MnO₂ (for batteries) and Al₂ O₃ (for hightemperature gaskets) may be added to the fibril dispersion prior to matformation at up to 50 parts added solids per part of fibrils.

Reinforcing webs and scrims may be incorporated on or in the mats duringformation. Examples are polypropylene mesh and expanded nickel screen.

Methods of Improving the Stability of Assemblages

In order to increase the stability of the fibril assemblages, it ispossible to deposit polymer at the intersections of the assemblage. Thismay be infiltrating the assemblage with a dilute solution of polymercement and allowing the solvent to evaporate. Capillary forces willconcentrate the polymer at fibril intersections. It is understood thatin order to substantially improve the stiffness and integrity of theassemblage, only a small fraction of the fibril intersections need becemented.

EXAMPLES

The invention is further described in the following examples.

EXAMPLE I Preparation of a Porous Fibril Mat

A dilute dispersion of fibrils is used to prepare porous mats or sheets.A suspension of fibrils is prepared containing 0.5% fibrils in waterusing a Waring Blender. After subsequent dilution to 0.1%, the fibrilsare further dispersed with a probe type sonifier. The dispersion is thenvacuum filtered to form a mat, which is then oven dried.

The mat has a thickness of about 0.20 mm and a density of about 0.20gm/cc corresponding to a pore volume of 0.90. The electrical resistivityin the plane of the mat is about 0.02 ohm/cm. The resistivity in thedirection perpendicular to the mat is about 1.0 ohm/cm.

EXAMPLE II Preparation of a Porous Fibril Mat

A suspension of fibrils is prepared containing 0.5% fibrils in ethanolusing a Waring Blendor. After subsequent dilution to 0.1%, the fibrilsare further dispersed with a probe type sonifier. The ethanol is thenallowed to evaporate and a mat is formed. The mat has the same physicalproperties and characteristics as the mat prepared in EXAMPLE I.

EXAMPLE III Preparation of a Low-Density Porous Fibril Plug

Supercritical fluid removal from a well dispersed-fibril paste is usedto prepare low density shapes. 50 cc of a 0.5% dispersion in n-pentaneis charged to a pressure vessel of slightly larger capacity which isequipped with a needle valve to enable slow release of pressure. Afterthe vessel is heated above the critical temperature of pentane(Tc=196.60), the needle valve is cracked open slightly to bleed thesupercritical pentane over a period of about an hour.

The resultant solid plug of Fibrils, which has the shape of the vesselinterior, has a density of 0.005 g/cc, corresponding to a pore volumefraction of 0.997%. The resistivity is isotropic and about 20 ohm/cm.

EXAMPLE IV Preparation of EMI Shielding

A fibril paper is prepared according to the procedures of EXAMPLE I.Table I below sets forth the attenuation achieved at several paperthickness.

                  TABLE I                                                         ______________________________________                                        FIBRIL PAPER                                                                  EMI SHIELDING                                                                 ATTENUATION 30 MHz TO 1 GHz                                                   THICKNESS, INCHES (MM)                                                                          WEIGHT   ATTENUATION                                        ______________________________________                                        0.002 (0.5)       12 G/M.sup.2                                                                           27 Db                                              0.005 (.125)       30      37 Db                                              0.017 (.425)      120      48 Db                                              ______________________________________                                    

EXAMPLE V

A fibril mat prepared by the method of EXAMPLE I is used as an electrodein an electrochemiluminescence cell such as is described in PCT U.S.85/02153 (WO 86/02734) and U.S. Pat. Nos. 5,147,806 and 5,068,088. Whenthe voltage is pulsed in the presence of ruthenium trisbipyridyl,electrochemiluminescence is observed.

What is claimed is:
 1. A method of preparing a three-dimensional,macroscopic assemblage of a multiplicity of randomly oriented carbonfibrils, said fibrils being substantially cylindrical with asubstantially constant diameter, having c-axes substantiallyperpendicular to their cylindrical axis, being substantially free ofpyrolytically deposited carbon and having a diameter between about 3.5and 70 nanometers, said assemblage having a bulk density of from 0.001to 0.50 gm/cc, comprising the steps of:(a) dispersing a multiplicity ofsaid fibrils in a medium; and (b) separating said assemblage from saidmedium.
 2. A method as recited in claim 1 for the formation of a porousmat or sheet of said carbon fibrils, said mat or sheet having a bulkdensity of from 0.05 to 0.50 gm/cc comprising the steps of:(a)vigorously dispersing a multiplicity of said fibrils in water or anorganic solvent and thereby forming a dispersion, said dispersioncontaining less than 10.0 percent by weight of fibrils, and (b)filtering the dispersion to form said porous mat or sheet.
 3. A methodas recited in claim 2, wherein said mat or sheet has a thickness betweenabout 0.02 and about 0.50 millimeters.
 4. A method as recited in claim 1for the formation of a porous mat or sheet of said carbon fibrils, saidmat or sheet having a bulk density of from 0.05 to 0.50 gm/cc comprisingthe steps of:(a) vigorously dispersing a multiplicity of said fibrils inwater or an organic solvent and thereby forming a dispersion, saiddispersion containing less than 10.0 percent by weight of fibrils; and(b) permitting the water or organic solvent to evaporate from saiddispersion thereby leaving said porous mat or sheet.
 5. A method asrecited in claim 4, wherein said mat or sheet has a thickness betweenabout 0.02 and about 0.50 millimeters.
 6. A method as recited in claim 1for the formation of a low-density porous plug of said carbon fibrils,said plug having a bulk density of from 0.001 to 0.05 gm/cc comprisingthe steps of:(a) forming a gel or paste comprising a multiplicity ofcarbon fibrils in a fluid, said gel or paste comprising less than 10.0percent by weight of fibrils; (b) heating said gel or paste in apressure vessel to a temperature above the critical temperature of saidfluid; (c) removing supercritical fluid from said pressure vessel; and(d) removing said porous plug from said pressure vessel.
 7. Athree-dimensional, macroscopic assemblage of randomly oriented carbonfibrils prepared by a method of claim
 1. 8. A method as recited in claim1, wherein the step of dispersing forms a dispersion containing fromabout 0.01 to about 10% fibrils in said medium.
 9. A method as recitedin claim 1, further comprising dispersing non-fibril particulate solidsin said medium.
 10. A method as recited in claim 9, wherein saidparticulate solids are selected from the group consisting of from MnO₂and Al₂ O₃.
 11. The method as recited in claim 9, wherein saidparticulate solids are added up to about 50 parts particulate solids perpart of fibrils.
 12. The method as recited in claim 9, wherein saidparticulate solid is an electroactive material.
 13. The method asrecited in claim 9, wherein said solid particulate is a catalyticallyactive material.
 14. The method as recited in claim 9, wherein solidparticulate is lead or a lead compound or manganese or a manganesecompound.
 15. The method as recited in claim 1, further comprising thestep of incorporating a mesh on or in the assemblage during formation.16. The method as recited in claim 1, wherein the step of dispersingsaid fibrils comprises ultrasonication.
 17. A method of preparing acomposite of (a) a three-dimensional, macroscopic assemblage of amultiplicity of randomly oriented carbon fibrils, said fibrils beingsubstantially cylindrical with a substantially constant diameter, havingc-axes substantially perpendicular to their cylindrical axis, beingsubstantially free of pyrolytically deposited carbon and having adiameter between about 3.5 and 70 nanometers, said assemblage having abulk density of from 0.001 to 0.50 gm/cc, and (b) a second material,comprising the steps of:(a) dispersing a multiplicity of said fibrilsand a second material in a medium; and (b) separating said compositefrom said medium.
 18. A composite prepared by a method of claim
 17. 19.A method of preparing a three-dimensional, macroscopic assemblage of amultiplicity of randomly oriented carbon fibrils, said fibrils beingsubstantially cylindrical with a substantially constant diameter, havingc-axes less than substantially perpendicular to their cylindrical axis,being substantially free of pyrolytically deposited carbon and having adiameter between about 3.5 and 70 nanometers, said assemblage having abulk density of from 0.001 to 0.50 gm/cc, comprising the steps of:(a)dispersing a multiplicity of said fibrils in a medium; and (b)separating said assemblage from said medium.
 20. A method of preparing athree-dimensional, macroscopic assemblage of a multiplicity of randomlyoriented carbon fibrils, said fibrils being substantially free ofpyrolytically deposited carbon, said assemblage having a bulk density offrom 0.001 to 0.50 gm/cc comprising the steps of:(a) dispersing amultiplicity of said fibrils in a medium; and (b) separating saidassemblage from said medium.